1. Field of the Invention
The present invention relates generally to CO2 extraction and, more specifically, to recovery of CO2 from seawater and/or aqueous bicarbonate systems using a multi-layer gas permeable membrane.
2. Description of the Prior Art
There is interest in producing synthetic fuel from renewable sources on sea-based vessels to avoid the risks in procuring fuel from foreign sources and/or in maintaining long supply lines. The procurement and transportation risk can be reduced by producing synthetic fuel from hydrogen and carbon generated near the point of fuel use. Although there are current technologies to synthesize hydrocarbon fuel on land given sufficient primary energy resources such as coal, these technologies are not practical for sea-based generation.
The ocean is a possible resource for carbon dioxide. The total carbon content of the world's oceans is roughly 38,000 GtC (gigaton of carbon). Over 95% of this carbon is in the form of dissolved bicarbonate ion (HCO3−). This ion along with carbonate is responsible for buffering and maintaining the ocean's pH, which is relatively constant below the first 100 meters. In addition, the bicarbonate/carbonate system maintains the pH of the ocean below 100 m. This dissolved bicarbonate and carbonate is essentially bound CO2, and as shown in equation (1), the sum of these species along with gaseous CO2 represents the total carbon dioxide concentration [CO2]T of seawater.[CO2]T=[CO2(g)]+[HCO3−]+[CO32−]  (1)
At a typical ocean pH of 7.8, [CO2]T is about 2000 μmoles/kg near the surface and 2400 μmoles at depths below 300 meters. (Takahasi et al., “The Alkalinity and Total Carbon Dioxide Concentration in the World Oceans,” Carbon Cycle Modeling, John Wiley and Sons, New-York, 271-286 (1981); Takahasi et al., “Carbonate Chemistry of the Surface of the Waters of the World Oceans,” Isotope Marine Chemistry, Uchida Rokakuho, Tokyo, Japan, 291-326 (1980), the entire contents of each are incorporated herein by reference.) This equates to approximately 100 mg/L of [CO2]T of which 2 to 3% is CO2 (g), 1% is carbonate, and the remainder is dissolved bicarbonate.
In the atmosphere, the concentration of [CO2]T approximately is 370 ppm (v/v), which is 0.7 mg/L (w/v) or 780 GtC. Comparing this value on a w/v basis, it is apparent that CO2 in seawater is about 140 times greater than air. (Coffey et al., “Hydrogen as a Fuel for DOD,” Defense Horizons, 36, 1 (2003), the entire contents of which is incorporated herein by reference.) Thus if carbon dioxide could be economically and efficiently extracted from the ocean, then marine engineering processes such as OTEC (ocean thermal energy conversion) (Mohanasundaram, “Renewable Power Generation-Utilising Thermal Energy from Oceans,” Enviro. Sci. Eng., 4, 35 (2007), the entire contents of which is incorporated herein by reference) could be proposed to utilize this carbon as a chemical feedstock in processes such as catalytic polymerization with hydrogen.
Seawater is a very complex buffered system that is in equilibrium with the atmosphere above it. Thus under equilibrium conditions, the dissolved [CO2(g)] in equation (1) is actually hydrated with one mole of water in the form known as carbonic acid as shown in equation (2). The dissolved bicarbonate and carbonate are in equilibrium with this dissolved carbonic acid species as shown in equation (3).
The carbonic acid species is in equilibrium with the gas phase above the water as carbon dioxide, and so any reduction of carbon dioxide partial pressure in the gas phase will cause the equilibria of the entire system to shift to the left (equation (3)). (Werner et al., Aquatic Chemistry: An introduction emphasizing chemical equilibrium in natural waters; Wiley-Interscience: New York (1970); Glade et al., “Modeling of CO2 release and the carbonate system in multiple-effect distillers,” Desalination, 222, 605 (2008), the entire contents of each are incorporated herein by reference.) Kinetically, this is a very slow and not well understood process. (Wilson, “A Renaissance for Hofmeister,” C&EN, 85, 47 (2007), the entire contents of which are incorporated herein by reference.) For example, it would take about 2.8 hours to completely remove all the carbonate species in the form of CO2 gas in one liter of seawater by applying a vacuum of 15 mm of Hg to the seawater spread in a thin film (such as in a rotary evaporator). (Werner et al., Aquatic Chemistry: An introduction emphasizing chemical equilibrium in natural waters; Wiley-Interscience: New York (1970).) Even a bulk process such as vacuum degassing of carbon dioxide gas would yield only about 0.1 kg/sec of carbon dioxide. From an ocean engineering perspective, the rate of carbon recovery by vacuum extraction or vacuum degassing of the 2 to 3% of the dissolved CO2 would be entirely too slow to obtain useful amounts of carbon on the order of 10 kg/sec.
In addition to the complex equilibrium buffer system affecting CO2 recovery from seawater, there is a potential for the excessive salinity of seawater to interfere with developing fast and efficient approaches to CO2 capture. The salinity is on the order of 35 g/L and is attributed primarily to sodium chloride. The chloride content is approximately 240 times more concentrated than that of bicarbonate. In efforts to overcome these challenges, the use of simple ion exchange resin systems as direct and indirect methods of CO2 recovery from seawater has been investigated. (Hardy et al., Extraction of Carbon Dioxide from Seawater by Ion Exchange Resin Part I: Using a Strong Acid Cation Exchange Resin, Memorandum Report 6180-07-9044, Naval Research Laboratory: Washington D.C., Apr. 20, 2007, the entire contents of which are incorporated herein by reference.) Strong base anion exchange resins' selectivity and capacity was proven inefficient to acquire the 10 kg/sec of carbon needed for a catalytic process. Strong acid cation exchange resins were successful at acidifying seawater to a pH less than 6 so the total CO2 existed in the dissolved gas form. (Johnson et al., “Coulometric TCO2 Analyses for Marine Studies: An Introduction,” Marine Chem., 16, 61 (1985), the entire contents of which are incorporated herein by reference.) However, that approach was deemed impractical from a regeneration perspective.
Gas permeable membranes are available commercially for the removal or addition of gases to liquids. Most of these applications are near atmospheric pressure and include water purification, blood oxygenation and artificial lung devices. (Bhaumik et al., “Hollow Fiber Membrane Degassing in Ultrapure Water and Microbiocontamination,” J. Membr. Sci, 235, 31 (2004); Lund et al., “Gas Permeance Measurement of Hollow Fiber Membranes in Gas-Liquid Environment,” AIChE J., 48, 635 (2002); Lund et al., “Gas Permeability of Hollow Fiber Membranes in a Gas-Liquid System,” J. Membr. Sci, 117, 207 (1996); and Eash et al., “Evaluation of Plasma Resistant Hollow Fiber Membranes for Artificial Lungs,” ASAIO J., 50, 491 (2004), the entire contents of each are incorporated herein by reference.) However, some are operated at higher pressures such as beverage carbonation. (Bosko, “Hollow Fiber Carbonation,” U.S. Pat. No. 6,712,342, Mar. 20, 2004; Gabelman et al., “Hollow Fiber Membrane Contactors,” J. Membr. Sci, 159, 61 (1999), the entire contents of each are incorporated herein by reference.) It is well known that these membranes work on the principle of dissolved gases such as carbon dioxide diffusing across the membrane through the pores as a function of differential partial gas pressures. Therefore, it has been assumed for gas/liquid systems that only the dissolved carbon dioxide gas is removed while the bound carbon dioxide in the ionic form of bicarbonate and carbonate is not involved. (Wiesler et al., “Deaeration: Degasification of Water using Novel Membrane Technology,” Ultrapure Water, UP130653, 53-56 (1996), the entire contents of which are incorporated herein by reference.) The rate and yield of carbon dioxide for a given separation is dependent on several parameters which include the degasification matrix, membrane material and geometry, temperature, and pressure. (Matson et al., “Review Article Number 13: Separation of Gases with Synthetic Membranes,” Chem. Eng. Sci., 38, 503 (1983), the entire contents of which is incorporated herein by reference.)